Electrolytic separation of metals

ABSTRACT

A process for separating silver, gold or a silver/gold alloy from a composite metal body in which the silver, gold or alloy is adhered as an external layer over a ferritic or austenitic stainless steel substrate. Separation is effected by controlled potential electrolysis in which the metal composite serves as the anode and in which the electrolytic solution used contains between about 1% and about 10% by weight of an alkali metal cyanide and up to about 10% by weight of an alkali metal hydroxide. During electrolysis the anode voltage is controlled by reference to a standard electrode at a voltage at which the degree of dissolution of the stainless steel in the half cell comprising stainless steel and the solution is substantially less than the degree of dissolution of the silver, gold or silver/gold alloy in the half cell comprising silver, gold or silver/gold alloy and the solution thereby causing selective removal from the substrate of the silver, gold, or alloy thereof.

United States Patent 1 Baboian et a1.

[451 May 27, 1975 I ELECTROLYTIC SEPARATION OF METALS [73] Assignee:Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Dec. 12, 1973 [21] Appl. No.: 423,948

[52] US. Cl 204/146; 204/140 [51] Int. Cl. C23b 1/00; BOlk 3/00 [58]Field of Search 204/146, 140

[56] References Cited UNITED STATES PATENTS 2,735,810 2/1956 Gagliano204/146 3,826,724 7/1974 Riggs, Jr. et a1. 204/146 OTHER PUBLICATIONSMetal Cleaning and Finishing, January 1937, pg. 31. Metal Finishing,November 1945, pp. 457 and 458. Corrosion, Vol. 16, No. 2, February1960, pgs. 4754. Plating, October 1948, pgs. 1013 and 1044.

Primary ExaminerT. M Tufariello Attorney, Agent, or Firm-James P.McAndrews; John A. Haug; Edward J. Connors, Jr.

[57] ABSTRACT A process for separatingsilver, gold or a silver/goldalloy from a composite metal body in which the silver, gold or alloy isadhered as an external layer over a ferritic or austenitic stainlesssteel substrate. Separation is effected by controlled potentialelectrolysis in which the metal composite serves as the anode and inwhich the electrolytic solution used contains between about 1% and about10% by weight of an alkali metal cyanide and up to about 10% by weightof an alkali metal hydroxide. During electrolysis the anode voltage iscontrolled by reference to a standard electrode at a voltage at whichthe degree of dissolution of the stainless steel in the half cellcomprising stainless steel and the solution is substantially less thanthe degree of dissolution of the silver, gold or silver/gold alloy inthe half cell comprising silver, gold or silver/gold alloy and thesolution thereby causing selective removalfrom the substrate of thesilver, gold, or alloy thereof.

10 Claims, 3 Drawing Figures I l I 29 fluff/WM: M575? fiiddkDE/Q L (0 EI I W;

M144 l I HMPL/F/ER 35) 1 I I I I 5 1 12 2 fla e aye I fern/N42 amt/r601.

I za/vmaa 47 VI/ nwz/z/m \a I I I :(JRR'A/T 45 Merl/4e I 5 I EKG/Q55 iporn/rm I (alt r402 I 557' Pd/NT I Ao/usr/wA/r I I I I I FOWiEELECTROLYTIC SEPARATION OF METALS BACKGROUND OF THE INVENTION Thisinvention relates to electrolytic separation of metals and moreparticularly to a controlled potential electrolytic method forselectively removing a gold, silver or gold/silver alloy external layerfrom a metal composite body having a substrate constituted by a ferriticor austenitic stainless steel.

Metal laminates or composites are widely used in a variety of differentindustrial and commercial applications. To produce such composites, acoating or cladding of one metal is applied to a substrate constitutedby a different metal using various techniques. including vapordeposition, roll bonding, electroplating, etc. Both in the coating orcladding processes and in the fabrication of metal products from themetal composite stock, some amount of off-grade or scrap composite metalis inevitably produced. To conserve raw materials and minimize the costof both the composite metal and products produced therefrom, it ishighly desirable to recover the components of the scrap composite andreuse these components in further coating or cladding operations.Recovery of the scrap necessarily involves separation of the externallayer of metal from the substrate metal. One technique which has beenused commercially to accomplish this separation'is vaporization of oneof the metals, normally that constituting the coating or cladding.vaporization techniques, however, require high temperatures and carefulcollection of vapors to avoid loss of metal vapor to the surroundings, aconsideration which may be especially critical where the cladding metalis expensive, toxic or both. Vaporization recovery techniques havegenerally proved to be both costly and inefficient.

Chemical separation methods have also been used in the art to removemetal coatings or claddings from a substrate. Chemical separations arefrequently unattractive, however, since it is difficult to devisesolvent systems which will selectively dissolve one metal of a compositewithout at least partially dissolving the other. The consequent processcomplications, product purity problems, and cost of chemical separationare thus apparent.

A potential variant of chemical separation is separation byelectrochemical processes in which the metal composite serves as ananode and one metal is preferentially removed by application ofelectrolytic current. However, electrolytic processes in general sufferfrom the same limitations as wet chemical procedures in that anodicdissolution is not normally selective, especially where the metals ofthe composite have oxidation potentials which are not substantiallydifferent. Moreover, where the metal to be removed is one normallyconsidered to have substantially greater nobility than the substratefrom which it is removed, electrochemical techniques might beanticipated to be especially unattractive.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a process for substantially quantitative recovery of gold,silver or a gold/silver alloy from a metal composite body in which thegold, silver or alloy is present as an external layer on a substrateconstituted by a ferritic or austenitic stainless steel. It is also anobject of the present invention to provide such a method in whichremoval is rapidly, efficiently and economically accomplished withoutsignificant attack, degradation or deterioration of the substrate. It isa further object of the present invention to provide such a method inwhich the gold, silver or alloy is recovered in pure form substantiallyfree of the metals of the substrate. A particular object of the presentinvention is to provide such a method in which separation is effected byelectrochemical means. Other objects and features will be in partapparent and in part pointed out hereinafter.

Briefly, the present invention is directed to a process for separating afirst metal selected from the group consisting of silver and gold from acomposite metal body in which said first metal is adhered as an externallayer over a substrate comprising a second metal selected from the groupconsisting of austenitic and ferritic stainless steels. In this process,the metal composite body is immersed in a solution containing betweenabout 1 and about 10% by weight of an alkali metal cyanide and up toabout 10% by weight of an alkali metal hydroxide. Direct current issupplied to the body from the positive terminal of a direct currentpower source whose negative terminal is connected to a second electrodeimmersed in the electrolytic solution, thus establishing an electrolyticcircuit in which the body is the anode and the second electrode is thecathode. The voltage at the anode is controlled by reference to astandard electrode at a voltage at which the degree of dissolution ofthe stainless steel in the half-cell comprising the second metal and thesolution is substantially less than the degree of dissolution of thefirst metal in the half-cell comprising the first metal and the solutionthereby causing selective removal of the first metal from the body.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graph illustrating aspectsof methods of this invention and showing the potentiostatic polarizationcurves for Type 430 stainless steel in various electrolytic solutions;

FIG. 2 is a similar graph showing the polarization curves for gold,silver and Type 430 stainless steel in a solution containing 5% sodiumcyanide and 5% sodium hydroxide; and

FIG. 3 is a schematic diagram of apparatus suitable for practicing theinstant invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the presentinvention, it has now been discovered that controlled potentialelectrolysis may be utilized to remove an external layer of silver, goldor a gold/silver alloy from a metal composite in which the substrate isa relatively less noble metal such as a ferritic or austenitic stainlesssteel. At certain anode voltages in a particular electrolytic solution,differential current densities at the gold/silver anode surface and thestainless steel anode surface favors the preferential dissolution of theformer, and relative passivation of the latter. This phenomenon isillustrated by the polarization curves set forth in FIG. 2.

At an anode constituted by a particular metal immersed in a particularelectrolytic solution, current density generally tends to increase asthe absolute potential applied at that anode is made less negative.However, due to the passivity resulting from polarization at certainanode potentials, the current density curve for some metals, includingaustenitic and ferritic stainless steels, passes through minima at suchpotentials. At other voltages current density maxima may be observed.Minima on the polarization curve are fre quently attributable to theformation of passivating films at particular voltages. Diffusionlimitations on .ansport of the reactants ant. zt tltzsts tit" tn. anodicreaction may also contribute to minimum current density.

As indicated in Fit]. 2, it has been found that at certain voltagesstainless steels are more highly passivated than either gold or silver.Electrolysis in which the anode is controlled at such voltages affordspreferential anodic dissolution of gold and/or silver and thus providesa uniquely advantageous method for the recovery of these metals fromscrap composites in which they are present as coatings or claddings onstainless steel substrates.

in the process of the present invention, a composite metal body havingan external layer of gold, silver or gold/silver alloy over a stainlesssteel substrate is immersed in an electrolytic solution containingbetween about 1 and about 10% by weight of an alkali metal cyanide andup to about l% by weight of an alkali metal hydroxide. A solutioncontaining on the order of by weight sodium cyanide and 5% by weightsodium hydroxide is preferred.

The metal body is electrically connected to the positive terminal of adirect current power source whose negative terminal is connected to acathode which is also immersed in the electrolytic solution. Anelectrolytic circuit is thus established in which the composite metalbody is the anode. The cathode is preferably constructed of a materialwhich affords a half-cell reaction resulting in the deposition thereonof the gold and/or silver dissolving at the anode. Conveniently, thecathode is constructed of metal of the same composition as that beingrecovered.

Electrolysis is effected by supply of direct current from the powersource to the electrodes immersed in the solution. To provide selectiveremoval of silver andor gold from the stainless steel substrate, thevoltage at the anode is controlled at a voltage at which the degree ofdissolution of the stainless steel in the half-cell comprising stainlesssteel and the electrolytic solution is substantially less than thedegree of dissolution of the silver, gold or silver/gold alloy in thehalf-cell comprising silver/gold and the electrolytic solution. Thisvoltage is selected from polarization curves of the type shown in FIGS.1 and 2. A low degree of dissolution is indicated by a low currentdensity at a particular voltage, while a high degree of dissolutionresults in a high current density with a consequently high rate of metalremoval. The operating voltage selected should be one where the currentdensity for the stainless steel substrate is at a minimum, while thecurrent density for silyer/gold is reasonably high. For maximumselectivity, the ratio between the current densities at the voltage ofchoice should be as high as possible. It is particularly important thatthe current density ratio be high where the area of stainless steelsubstrate metal exposed to the electrolytic solution exceeds the exposedarea of the gold/silver external layer. A difference between the currentdensities of two or more orders of magnitude will almost always provideclean separation. Where the exposed area of the external layer exceedsthat of the substrate. lower ratios may also provide good separation.

As used in this disclosure, the term external layer of a metal compositesimply means a layer which is exposed to the electrolytic solution foranodic dissolution. If the metal composite has two layers, the substrateto which the external layer is adhered will normally be exposed to thesolution also, but the electrolyte and voltage are chosen to suppressits dissolution. The composite may also have the substrate sandwichedbetween two or more cladding layers. It will be understood that, in thepresent context, the terms external layer and -substrate carry noimplication as to which metal was applied to which in the initialpreparation of the composite.

To control the voltage during electrolytic separation operations, astandard electrode is immersed in the electrolyte solution in closeproximity to the anode. The difference between the anode voltage andthat of the standard electrode is constantly measured and, in responseto this measurement, the voltage output of the power source iscontrolled to maintain the anode voltage at the predetermined voltagereferred to the standard electrode. As a result, selective dissolutionof the external layer of the metal composite is obtained.

As anodic dissolution of the external layer proceeds substantially tocompletion, the current density falls off to a low level and theseparation is complete. Essentially quantitative removal of the externallayer is thus achieved without significant attack on the substrate.Metal dissolved from the external layer at the anode deposits at thecathode in high purity. Where the cathode initially consists of thissame metal, the cathodic product is simply washed and dried, and maythen be suitable for use without further purification. Similarly, thedenuded anode provides a relatively pure source of the stainless steelsubstrate metal. The process of the invention thus provides a simple,direct, rapid and economical method for separation and recovery of theconstituent metals of the composite.

Apparatus for carrying out the process of the invention is illustratedin FIG. 3 and includes a potential measuring section and a potentialcontrol section. Shown at 1 is an anode constituted by a body of scrapmetal composite. Through a lead wire 3, anode 1 is electricallyconnected to ground as indicated at 5 and to the positive terminal of adirect current power source (power amplifier) 7 which has a controllablevariable output voltage. The negative terminal of power source 7 iselectrically connected to cathode 11 through a conductor 9, a currentmeter 13 and a current recorder 15, the latter indicating the rate ofelectrolysis and power consumption of the electrolytic circuit, while avoltmeter 17 indicates the output voltage level of the power amplifier.

The voltage of anode l is sensed by a reference electrode l9 placed inproximity to the anode. Each of the threeelectrodes is immersed in anelectrolytic solution 21 in a container 23. A signal lead 25 transmitsthe voltage of anode l to the input of a potentiometric control device27 and to the positive input terminal ofa potential amplifier 29, whilea signal lead 31 interconnects reference electrode 19 to one inputterminal of a control amplifier 33 and to the negative input terminal ofpotential amplifier 29. Potential amplifier 29 thus provides a signalcorresponding to an amplified difference in voltage between anode l andelectrode 19, and this differential voltage is indicated by a potentialmeter 35 and recorded by a potential recorder 37.

Potentiometric control means 27 comprises a balance circuit includingplus and minus temperature compensated zener diode regulated supplies 39and 41 and a potential control set point potentiometer 43. The otherinput terminal of amplifier 33 is connected by a lead 45 to the rotor orarm of potentiometer 43. As long as the voltage of an anode l differsfrom the voltage of standard electrode 19 by an amount corresponding tothe set point of potentiometric means 27, there will be no input errorsignal applied to the input of amplifier 33. However, upon the anodevoltage straying from the set point the resultant difference between thevoltages transmitted by leads 31 and 45 applies an error signal tocontrol amplifier 33 which in turn transmits a control signal throughline 47 to power amplifier 7 adjusting the total voltage output of thepower source to bring the voltage of anode 1 back to the desired level.When the voltage of the anode becomes too positive relative to thestandard, the output voltage of power source 7 is reduced to bring anode1 back to the control level and, when the voltage of anode 1 becomes tooEXAMPLE 3 Controlled potential electrolysis was carried out with ananode consisting of a one-inch square piece of type 430 stainless steelapproximately 0.005 in. thick having a 0.000] in. thick claddingconstituted by an alloy containing 75% gold and silver. The cathode wasa 1- inch square piece of nickel. Electrolysis was conducted in 50 mlofa 5% sodium cyanide/5% sodium hydroxide solution, using a potentiostatgenerally similar to that shown in FIG. 3 and described above. Such apotentiostat is available under the trade designation Duffers Model 600.The anode potential was controlled relative to a saturated calomelelectrode also immersed in the electrolytic solution. Runs were made atseveral different voltages, and during each run the solution was rapidlystirred by means of a magnetic stirrer. At the end of each run, theanode was analyzed for silver and gold, while the cathode and theelectolytic solution were analyzed for iron and chromium. The results ofthese runs are set forth in the following table.

CONTROLLED POTENTIAL ELECTROLYSIS OF 75% Au-25% Ag/430 SS SCRAP INSODIUM CYANIDE SOLUTION Analysis of Electrolytic Controlled StrippingTime Solution Potential (Minutes) Analysis of Anode Analysis of CathodeFe (ppm) Cr (ppm) of Scrap Au Ag Fe Cr .(V vs. SCE**) 0.50 240 0.00250.0005 None None I 8 1.0 0.30 60 0.0 0.0l4 0.00l 0.009 None L9 2.2 +0.2030 0.013 0.00l 0.009 None 1.7 2.3 +0.30 20 0.0025 0.0002 0.001 None 3.51.4 +0.30 60* 0.0024 0.0002 0.0003 None 3.6 2.3 +0.30 120* 0.0025 0.00040.00l 0.0002 3.4 2.5 +0.40 l500 +0.70 SS Dissolves Only 20 MinutesRequired "Saturated calomel electrode negative, the total output ofpower source 7 is increased to reestablish the control level.

The following examples illustrate the invention.

EXAMPLE 1 Polarization curves were obtained for 430 stainless steel in 1N. sulfuric. 5% by weight sodium chloride so lution, and a solutioncontaining 5% by weight sodium cyanide and 5% by weight sodium hydroxiderespectively. To obtain the polarization curves, a 430 stainless steelanode having a surface area of 2 cm a large platinum cathode, and asaturated calomel sensing elec trode were immersed in 50 ml of theelectrolytic solution at ambient temperature. Using a variable voltagesource (such as that obtainable under the trade designation BeckmanElectroscan), the anode potential was scanned at a rate of 8 v/hr. andcurrent was measured as a function of voltage. The curves thus obtainedare set forth in FIG. 1.

EXAMPLE 2 Using the technique described in Example 1, polarizationcurves were obtained at ambient temperature for silver and gold in asolution containing 5% sodium cyanide and 5% sodium hydroxide. Thecurves obtained are set forth in FIG. 2 which also includes a curve for430 stainless steel essentially the same as that shown in FIG. 1.

As the above table indicates, optimum voltage for stripping a gold/25%silver alloy from a stainless steel substrate is on the order of +0.30 vrelative to a saturated calomel electrode.

Where the anode used has a pure gold external layer on a 430 stainlesssteel substrate, pure gold may be selectively stripped at a voltage ofapproximately 0.5 v or approximately +0.3 v versus the saturated calomelelectrode. If the anode has a pure silver external layer on a 430stainless steel substrate, silver may be selectively stripped atpotentials between about 0.6 v and +0.5 v versus the saturated calomelelectrode. The optimum potential for silver removal is approximately 0.0

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above methods without departingfrom the scope of the invention, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process for separating a first metal selected from the groupconsisting of silver, gold and alloys of silver and gold from acomposite metal body in which said first metal is adhered as an externallayer over a substrate constituted by a second metal selected from thegroup consisting of ferritic and austenitic stainless steels. theprocess comprising the steps of:

immersing the body in a solution containing between about 1 and about10% by weight of an alkali metal cyanide and up to about 1071 by weightof an alkali metal hydroxide; supplying direct current to said body fromthe positive terminal of a direct current power source whose negativeterminal is connected to a second electrode immersed in said solutionthereby establishing an electrolytic circuit in which said body is theanode and said second electrode is the cathode;

arranging a saturated calomel standard electrode ad jacent saidcomposite metal body for continuously determining the voltage of saidanode relative to said standard electrode; and

continuously regulating the voltage applied between said anode andcathode for maintaining the voltage of said anode relative to saidstandard electrode within the range from 0.6 to +0.5 volts for causingselective removal of said first metal from said body.

2. A process as set forth in claim 1 wherein said first metal issubstantially pure gold and said voltage is controlled at approximately0.5 volts relative to a saturated calomel standard reference electrode.

3. A process as set forth in claim 1 wherein said first metal issubstantially pure gold and said anode voltage is maintained at a levelof approximately +0.3 volts relative to a saturated calomel standardreference electrode.

4. A process as set forth in claim 1 wherein said first metal issubstantially pure silver and said voltage is maintained at a level ofbetween about -().6 and about +0.5 volts relative to a saturated calomelstandard reference electrode.

5. A process as set forth in claim 1 wherein said first metal issubstantially pure silver and wherein the voltage of said anode relativeto said standard calomel electrode is maintained at a level ofapproximately 0.0 volts.

6. A process as set forth in claim I wherein said first metal is agold/silver alloy and said voltage is maintained at a level ofapproximately +0.3 volts relative to said saturated calomel standardreference electrode.

7. A process as set forth in claim 1 wherein said electrolyte solutioncontains on the order of 5% by weight sodium cyanide and on the order of5% by llveight sodium hydroxide.

8. A process as set forth in claim 1 wherein the volt- F age of saidanode relative to said standard electrode is controlled by controllingthe voltage outpui of said direct current power source automatically inresponse to the difference in voltage between said anoqil e and saidstandard electrode. 1

9. A process as set forth in claim 8 wherein said first metal isdeposited at said cathode.

10. A process as set forth in claim 9 wherein the electrolytic solutionis agitated during electrolysis.

1. A PROCESS FOR SEPARATING A FIRST METAL SELECTED FROM THE GROUPCONSISTING OF SILVER, GOLD AND ALLOYS OF SILVER AND GOLD FROM ACOMPOSITE METAL BODY IN WHICH SAID FIRST METAL IS ADHERED AS AN EXTERNALLAYER OVER A SUBSTRATE CONSTITUTED BY A SECOND METAL SELECTED FROM THEGROUP CONSISTING OF FERRITIC AND AUSTENITIC STAINLESS STEELS, THEPROCESS COMPRISING THE STEPS OF: IMMERSING THE BODY IN A SOLUTIONCONTAINING BETWEEN ABOUT 1 AND ABOUT 10% BY WEIGHT OF AN ALKALI METALCYANIDE AND UP TO ABOUT 10% BY WEIGHT OF AN ALKALI METAL HYDROXIDE;SUPPLYING DIRECT CURRENT TO SAID BODY FROM THE POSITIVE TERMINAL OF ADIRECT CURRENT POWER SOURCE WHOSE NEGATIVE TERMINAL IS CONNECTED TO ASECOND ELECTRODE IMMERSED IN SAID SOLUTION THEREBY ESTABLISHING ANELECTROLYTIC CIRCUIT IN WHICH SAID BODY IS THE ANODE AND SAID SECONDELECTRODE IS THE CATHODE; ARRANGING A SATURATED CALOMEL STANDARDELECTRODE ADJACENT SAID COMPOSITE METAL BODY FOR CONTINUOUSLYDETERMINING THE VOLTAGE OF SAID ANODE RELATIVE TO SAID STANDARDELECTRODE; AND CONTINUOUSLY REGULATING THE VOLTAGE APPLIED BETWEEN SAIDANODE AND CATHODE FOR MAINTAINING THE VOLTAGE OF SAID ANODE RELATIVE TOSAID STANDARD ELECTRODE WIHTIN THE RANGE FROM -0.6 TO +0.5 VOLTS FORCAUSING SELECTIVE REMOVAL OF SAID FIRST METAL FROM SAID BODY.
 2. Aprocess as set forth in claim 1 wherein said first metal issubstantially pure gold and said voltage is controlled at approximately-0.5 volts relative to a saturated calomel standard reference electrode.3. A process as set forth in claim 1 wherein said first metal issubstantially pure gold and said anode voltage is maintained at a levelof approximately +0.3 volts relative to a saturated calomel standardreference electrode.
 4. A process as set forth in claim 1 wherein saidfirst metal is substantially pure silver and said voltage is maintainedat a level of between about -0.6 and about +0.5 volts relative to asaturated calomel standard reference electrode.
 5. A process as setforth in claim 1 wherein said first metal is substantially pure silverand wherein the voltage of said anode relative to said standard calomelelectrode is maintained at a level of approximately 0.0 volts.
 6. Aprocess as set forth in claim 1 wherein said first metal is agold/silver alloy and said voltage is maintained at a level ofapproximately +0.3 volts relative to said saturated calomel standardreference electrode.
 7. A process as set forth in claim 1 wherein saidelectrolyte solution contains on the order of 5% by weight sodiumcyanide and on the order of 5% by weight sodium hydroxide.
 8. A processas set forth in claim 1 wherein the voltage of said anode relative tosaid standard electrode is controlled by controlling the voltage outputof said direct current power source automatically in response to thedifference in voltage between said anode and said standard electrode. 9.A process as set forth in claim 8 wherein said first metal is depositedat said cathode.
 10. A process as set forth in claim 9 wherein theelectrolytic solution is agitated during electrolysis.